The invention relates generally to treatment of wastes in soil and groundwater, and more specifically to in situ formation of apatitic compounds that selectively trap and contain radionuclides and heavy metals.
Leakage of radioactive materials and heavy metals from storage tanks and subsequent migration away from the containment area continues to be a significant unresolved problem at a number of government and private storage sites. Various techniques are used to try to isolate leaking storage containers and contaminated soil to prevent movement of contaminants into uncontaminated soil and especially groundwater.
One approach is simply to attempt to dig up and remove the contaminated soil. This, however, is costly, and disturbance of contaminated soil carries the risk that some contaminants will be missed or released and left to migrate further. Excavation also has a negative effect on soil stability. Excessive digging and excavation around waste tanks, for example, has the potential to aggravate waste transport by damaging heavily corroded containment drums and disturbing already contaminated soil.
Another approach is to establish an impermeable barrier or seal in the soil of a contaminated site in order to prevent migration of contaminants beyond the barriers. Barriers of this sort that are in use at various sites around the United States and abroad include vertical sleeves of steel or plastic placed in trenches surrounding a site. They also include walls formed through the injection of highly pressurized cementations grout in holes drilled in the soil. Emplacement of such barriers typically requires greatly disturbing the soil and often there is no convenient way to create a xe2x80x9cfloorxe2x80x9d or continuous barrier beneath the leaking tank or contaminated region. Consequently, the sequestration of the contaminants is incomplete and contaminants continue to migrate downward and may thereafter migrate outward. For areas under waste tanks, waste trenches and certain geological formations, forming a continuous impermeable barrier or seal is difficult and sometimes impossible.
Another approach is to create a permeable, chemically reactive barrier or zone that selectively actively attracts and chemically binds, sorbs, or traps contaminants (i.e., sequestration), while allowing water and other components or contaminants to pass through unaffected.
These chemically reactive materials can be combined with other components to form slurries that harden in the ground, forming semi-permeable reactive barriers. Jet injection processes, for example, are known and used wherein machines pump slurries in holes drilled around the perimeter of a leaking vessel or contaminated site. Additionally, trenches can be dug and backfilled using chemical sorbent materials. Each of these techniques, however, carries the disadvantages previously mentioned relating to significant disturbance of the soil and difficulty in fully surrounding (or encapsulating) a leaking waste tank or region of contaminated soil.
In situ formation of chemically reactive barriers or zones have shown considerable promise for removing certain radionuclides, heavy metals, and organic contaminants from soil and groundwater. An in situ barrier can have two parts: a permeable, chemically reactive zone that contains a material such as zero valent iron that sorbs radionuclides, and an impermeable section to funnel or direct groundwater into the permeable reactive zone. In situ permeable reactive barriers have advantages over other treatment technologies, such as pump and treat. These include the ability to treat large quantities of groundwater, easy retrieval of contaminants sorbed onto the reactive material, and lower cost in some instances. However, current in situ barrier designs can have problems with biofouling of the iron active media, variable porosity in the reactive zone that can cause groundwater to flow around or below the barrier zone, and relatively high cost.
Phosphate compounds, in general, can precipitate radionuclides (such as actinides) and heavy metals (lead, strontium, uranium, lanthanides) out of aqueous solution. Calcium phosphate compounds, and in particular, apatitic compounds (i.e., xe2x80x9capatitesxe2x80x9d), are very well suited to sequestering those contaminants. The mineral of human tooth enamel, dentin, and bone was identified as a calcium phosphate compound with an apatite structure as early as 1926 using x-ray diffraction. It has recently been discovered that apatitic compounds have an especially strong chemically affinity for radionuclides and heavy metals. Apatitic compounds have, generally, the chemical formula Ca5(PO4)3X, where X is a halide or hydroxyl. The term xe2x80x9capatitexe2x80x9d refers to a group of calcium phosphate compounds that share the same crystal structure. Apatitic compounds are chemically and morphologically similar to natural bone. A closely related mineral, hydroxyapatite, Ca10(PO4)6(OH)2, also found in human enamel, dentin, and bone, is also particularly effective at sequestering radionuclides and heavy metal contaminants (e.g., lead). Apatitic compounds and hydroxyapatite are very water insoluble, thermodynamically very stable, and naturally corrosion-resistant.
A number of studies over the past several decades have shown that radionuclides and heavy metals bind onto the surface of apatites and hydroxyapatite in such a fashion that they are almost totally immobilized. (Gauglitz, R, M. Holterdorf, W. Frank, and G. Marx, xe2x80x9cImmobilization of Actinides by Hydroxyapatite,xe2x80x9d Mat Res. Symp. Proc. Vol. 257 pp. 567-573 (1992); Jeanjean, J., J. C. Rouchaud, L. Tran, and M. Fedoroff, xe2x80x9cSorption of Uranium and Other Heavy Metals on Hydroxyapatite,xe2x80x9d Radioanal. Nuc. Chem. Letters, vol. 201(6) pp. 529-539 (1995); Arey, J. S., J. C. Seaman, and P. M. Bertsch, xe2x80x9cImmobilization of Uranium in Contaminated Sediments by Hydroxyapatite Addition,xe2x80x9d Environ. Sci. Technol. Vol. 33 pp. 337-342 (1999)). Consequently, apatitic compounds and hydroxyapatite have been used to trap, immobilize, and/or contain (i.e., sequester) radionuclides or heavy metals around contaminated sites and leaking storage containers.
Direct injection into the ground (or a container) of apatite is difficult at high concentrations, since apatite is a solid, water-insoluble material. Also, the diffusion or spreading of the apatite away from the injection site is limited by its chemical form, therefore requiring a larger number of injection holes.
In situ formation of apatite or hydroxyapatite can be accomplished via spontaneous conversion of brushite (CaHPO4xc2x7H2O) or other calcium phosphates, very slowly over time through the mechanism of hydrolysis. See Momma, H and T. Kamiya, xe2x80x9cPreparation of Hydroxyapatite by the Hydrolysis of Brushite,xe2x80x9d J. Mat. Sci. vol. 22 pp. 4247-4250 (1987); Boskey, A. L. and A. S. Posner xe2x80x9cFormation of Hydroxyapatite at Low Supersaturation,xe2x80x9d J. Physical Chem, vol. 80(1) pp. 40-45 (1976)).
More rapid in-situ formation of apatite or hydroxyapatite permeable reactive barriers or zones can be accomplished by separately injecting aqueous solutions of free phosphate and free calcium at different sites, as taught in U.S. application Ser. No. 09/516,481, which is incorporated herein by reference. By a variety of diffusion and migration mechanisms, the injected phosphate reagent mixes underground with the injected calcium reagent, then reacts to form the apatite or hydroxyapatite permeable reactive barrier or zone. Separate injection of the two different reagents at different sites allows for a larger volume of soil to be treated. Injection of the two reagents in the same hole could possibly plug up the hole (at high concentrations), or only treat a small volume (at lower concentrations), since the two reagents react quickly when in contact.
A wide variety of unidentifiable and identifiable insoluble calcium phosphate compounds can be formed by mixing calcium chloride (CaCl2) and potassium hydrogen phosphate (K2HPO4). These calcium phosphate compounds can be used to form relatively solid, water insoluble physical underground barriers that can slow or stop migration of contaminants. However, they will, in general, not sequester radionuclides and heavy metals because they have no chemical affinity for them.
Apatite, the most stable and least soluble subset of calcium phosphates known (and the most effective phosphate for sorbing radionuclides and heavy metals) is relatively difficult to form because it is not kinetically favored compared to the other unidentifiable and identifiable calcium phosphate compounds formed by mixing together calcium chloride (CaCl2) and potassium hydrogen phosphate (K2HPO4) or sodium phosphate (Na3PO4). Apatite, while not kinetically favored, is known to be thermodynamically favored to form. Consequently, apatite can be preferentially formed over the other undesirable forms of calcium phosphate if the pH, concentration, temperature, and soil conditions are appropriately controlled (to be discussed later).
A need remains, however, for a relatively rapid in situ method of forming a permeable reactive barrier or zone comprising apatitic compounds, including apatite and/or hydroxyapatite, where the method is capable of utilizing a single injection hole, and where the permeable reactive barrier or zone provides complete or near-complete containment and immobilization of radionuclide or heavy metal contaminants present in a large volume of contaminated soil or groundwater.
The present invention relates to methods for in situ formation in soil of a permeable reactive barrier or zone comprising a phosphate precipitate, such as apatite or hydroxyapatite, which is capable of selectively trapping and removing radionuclides and heavy metal contaminants from the soil, while allowing water or other compounds to pass through. A preparation of a phosphate reagent and a chelated calcium reagent is mixed aboveground and injected into the soil. Subsequently, the chelated calcium reagent biodegrades and slowly releases free calcium. The free calcium reacts with the phosphate reagent to form a phosphate precipitate. Under the proper chemical conditions, apatite or hydroxyapatite can form. Radionuclide and heavy metal contaminants, including lead, strontium, lanthanides, and uranium are then selectively sequestered by sorbing them onto the phosphate precipitate. A reducing agent can be added for reduction and selective sequestration of technetium or selenium contaminants.